When a tooth is damaged due to decaying and when a cavity is still relatively small, the tooth is directly restored by using a composite resin from the standpoint of aesthetic appearance, simplicity and quickness of operation. When the cavity is relatively large, on the other hand, the cavity is indirectly restored by using a prosthetic material prepared by using a metal, ceramics or a cured resin material.
The dental restorative such as a composite resin or a prosthetic material has no adhesiveness to the tooth. Therefore, the restorative is adhered to the tooth by using an adhesive which comprises a polymerizable composition (usually, a methacrylate-type monomer is a chief component). However, its adhering force to the tooth is not sufficient. For example, when the composite resin is used, the adhering strength that is obtained is not large enough to overcome the internal stress (tensile stress occurring in the interface between the tooth and the composite resin) that generates when the composite resin cures. In many cases, further, the adhering strength is not large enough to withstand the force produced by occlusion.
In order to improve the adhering strength of the adhesive, therefore, the tooth surface is pretreated in two steps, such as:    (a) a hard tooth (enamel comprising chiefly hydroxyapatite) is etched, and    (b) an adhesiveness-improving component called primer is permeated into the tooth,    at the time of using the adhesive.
Here, an aqueous solution of acid is usually used as a pretreating agent (pretreating agent for etching) of (a) above. Concretely, an aqueous solution of phosphoric acid, citric acid or maleic acid is used. Due to the treatment, the tooth is delimed, roughened, and the enamel or the dentin composed of spongy collagen fiber is exposed on the surface of the tooth.
To maintain a sufficiently large adhering strength, however, the adhesive component must permeate through the enamel exposed on the surface to enter into the dentin to a sufficient degree. For this purpose, the pretreatment (treatment with the primer) of (b) above is conducted. As the pretreating material (primer), there has been used an organic solvent of a hydrophilic monomer having excellent affinity to the tooth, such as hydroxyethyl methacrylate (HEMA) or the like. The primer by itself does not usually contain any polymerization initiator. However, the polymerizable monomer contained in the primer is polymerized and cured by the action of radicals formed by the adhesive at the time when the adhesive applied onto the primer undergoes the photo-curing reaction.
While the pretreatment is conducted as described above, there have been developed various dental adhesive compositions containing a polymerizable monomer having adhesiveness to the tooth in order to attain higher adhering strength and to reduce complexity of pretreating operation.
For example, a patent document 1 and a patent document 2 are proposing dental adhesive compositions containing an acidic group-containing polymerizable monomer as at least part of the polymerizable monomer component. These adhesive compositions exhibit higher adhering strengths since the acidic group-containing polymerizable monomer having an acidic group such as phosphoric acid group or carboxylic acid group in the molecules thereof exhibits a high affinity to the tooth (hydroxyapatite or collagen).
Further, patent documents 3 to 6 are proposing adhesive compositions in which an acidic group-containing polymerizable monomer is present together with water. These adhesive compositions exhibit both the etching function of the acid aqueous solution and the primer permeation accelerating function of the primer, and eliminate the need of separately applying a pretreating agent. Namely, these adhesive compositions can be used upon being applied only once, and can be advantageously used as adhesives (one-step type adhesives) featuring excellent operability. Further, the adhesive compositions containing the acidic group-containing polymerizable monomer and water can be used not only as adhesives but also as self-etching primers having the above-mentioned etching function and the permeation accelerating function (e.g., see patent document 7 and patent document 8).
As adhesives and self-etching primers having higher adhering strengths, there have, further, been known liquid adhesive compositions containing polyvalent metal ions obtained by being further blended with polyvalent metal ion-eluting fillers in addition to the acidic group-containing polymerizable monomer and water (e.g., see patent document 9 and patent document 10). Namely, with these adhesive compositions, not only the polymerizable monomer containing the acidic group-containing polymerizable monomer simply polymerizes but also the polyvalent metal ions ionically bond to the acidic groups of the acidic group-containing polymerizable monomer to form a reinforced structure. It is, therefore, presumed that the mechanical strength of the cured body is greatly enhanced.
Here, as the polyvalent metal ions, there can be exemplified alkaline earth metal ions and aluminum ions. As preferred acidic group-containing polymerizable monomers, further, there have been known those of the phosphoric acid type having an acidic group for their high acidity and strong affinity to the tooth. Among them, a polymerizable monomer having a hydrogenphosphoric diester group, such as bis(2-methacryyloyloxyethyl) hydrogenphosphate, is difunctional imparting not only a high acidity and affinity to the tooth but also imparting a crosslinked structure due to the chemical bond to the cured body making it possible to further improve the mechanical strength of the cured body, and is most desired. The difunctional polymerizable monomer having the hydrogenphosphoric diester group is, usually, used being mixed with a monofunctional acidic group-containing polymerizable monomer ((2-methacryloyloxyethyl)hydrogenphosphate, etc.)